The ever-increasing demand for cryogenic gases has led to a search for improved processes for separating the respective components of various gaseous mixtures, including air. Considerable investigation is being conducted in the area of semi-permeable polymeric membranes wherein such membranes exhibit selectivity toward the passage of one or more components of a gaseous mixture through the membrane.
Commercial applications for gas separation devices based on polymeric materials rely, in part, on maximizing the overall gas flux through the membrane. T. H. Kim, et al., J. Appl. Poly. Sci., 34 1767 (1987), report that membrane gas flux is related to the average space between the polymer chains. The investigators indicate that the density of the polymer can also be correlated to the overall gas flux.
The success of commercial gas separation applications utilizing polymeric membranes depends upon the identification of polymers having sufficiently high gas flux, high selectivity and good thermo-mechanical properties. High overall flux values are typically exhibited in polymers having low chain-chain interactions as exemplified by polymers such as poly(dimethylsiloxane) and poly(4-methyl-1-pentene). Materials having high gas flux values typically possess low glass transition temperatures (Tg) because of low chain-chain interactions in the polymeric material. As a consequence, these materials typically require special processing conditions to build in chemical and/or physiochemical crosslinking if such materials are to be employed in other than low application temperatures. In contrast, polymers having strong chain-chain interactions typically possess rather high Tg values and often exhibit rather low gas flux values.
Polyimides, which generally have strong chain-chain interactions and high Tg values, have been reported to exhibit rather high gas flux values when certain structural moieties are present. Specifically, U.S. Pat. No. 3,822,202 (1974); Re 30,351 (1980) discloses a process for separating fluids using a semi-permeable membrane formed from polyimides, polyesters or polyamides. The repeating units of the main polymer chain have at least one rigid divalent sub-unit, the two main chain single bonds extending therefrom which are not colinear, which are sterically unable to rotate 360.degree. around at least one of these bonds and have 50% or more of its main chain atoms as members of aromatic rings.
U.S. Pat. No. 4,705,540 discloses a highly permeable aromatic polyimide gas separation membrane and processes for using the membrane. The membrane is formed from an aromatic polyimide in which the phenylenediamine units are rigid and are substituted on all of the positions ortho to the amine nitrogen atoms and the acid anhydride units are essentially all attached to rigid aromatic moieties.
U.S. Pat. Nos. 4,717,393 and 4,717,394 teach polymeric membranes and processes for using such membranes for separating components of a gaseous mixture. The membranes disclosed in both of these patents are formed from semi-flexible, aromatic polyimides prepared by polycondensation of dianhydrides with phenylenediamines having alkyl substituents on all positions ortho to the amine functions, or with mixtures of other non-alkylated diamines, some components having substituents on all positions ortho to the amine functions. Membranes formed from this class of polyimides are stated to exhibit improved environmental stability and gas permeability due to optimization of the molecular free volume. Such membranes can also be photochemically crosslinked which in some instances results in a better performing semi-permeable membrane.
Attempts have been made to prepare membranes having high flux and high selectivity by creating a composite structure on the surface of the polymer by means of a chemical reaction between a labile polymer functionality and some "activating force". Such methods are taught in U.S. Pat. No. 4,657,564 wherein poly(1-trimethylsilylpropyne) is treated with a dilute fluorine gas stream and in U.S. Pat. No. 4,717,393 wherein a polyimide containing a benzophenone-containing linking group is irradiated with a medium pressure mercury lamp.
U.S. Pat. No. 4,838,900 discloses aromatic polyimides prepared by polycondensation of dianhydrides with methylene dianilines having substituents on all positions ortho to the amine functions to form membranes having high gas permeability. The gas permeability of the subject membranes is stated to increase substantially if structurally-rigid dianhydrides are used in combination with the substituted diamines.
Yamada and coworkers. Kobunshi Ronbunshu, 40(1) 35-40 (1983), disclose gas separating membranes formed of a polyimide, polysulfone or poly(ethylene terephthalate) which have been irradiated with ultraviolet light in the presence of a helium, oxygen or nitrogen atmosphere. Kapton H-type polyimide film produced by DuPont De Nemours, Wilmington, Del., which was irradiated by ultraviolet light in an atmosphere of air provided enhanced selectivity in the separation of light gases such as hydrogen and helium.
A need in the art exists for the identification of thin film polymers which provide enhanced selectivity for various gas separation applications while also possessing good mechanical properties and sufficient permeability.